The invention relates generally to methods and apparatus for coating articles, and more specifically to methods and apparatus for coating articles with oxide ceramic.
Oxide ceramic coatings have been traditionally applied using a plasma spray process. Oxide ceramic powder is injected into and melted by a high-temperature plasma plume or flame and the molten droplets are directed onto a metallic substrate or intermediate bond coat. Many applications only require a generic coating of a minimum or maximum thickness to protect the underlying substrate. For such applications, a plasma spray process is generally appropriate. For some applications which require additional strain tolerance such as ceramic rotor coatings that provide a sealing surface for cantilevered compressor vanes, an alternative process has been used in order to impart the special coating properties required for the application. That alternative process is combustion flame spray. Combustion flame spray operates at a reduced temperature compared to a plasma plume. The flame spray plume is just hot enough to partially melt the ceramic and provide the desired low level of densification and bonding within the coating. However, the combustion flame spray process is not particularly appropriate for tight control and repeatability, particularly when approaching the limits of the process. Thus, for applications like aerospace components, the oxide ceramic coatings cannot be applied consistently or uniformly using combustion flame spray.
Other applications require very fine control of the application process to achieve a satisfactory microstructure and physical properties on the component. Though plasma spray offers greater control and repeatability for application of oxide ceramic coatings, the plasma temperatures far exceed those required for certain oxide powders, such as alumina. Applying those lower temperature powders using a traditional plasma spray process results in too much heat flux to the powder and can negatively impact the desired microstructure, and thus the short- and long-term performance of the coated part.